Modulation system



v Patented Sept. 1Q, 1940 I UNITED STATES PATENT OFFICE 2,214,573 MODULATION SYSTEMY James D. Booth, Longmeadow, Mass, assignor-to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 17, 1938, Serial No. 191,012

9 Claims. (01. 179-1715) This invention relates to radio transmitters and plitude and the negative peaks are subtracted more particularly to arrangements employing. therefrom. In a similar manner when the moduh level modulation. lation is applied to the plate circuit of an am- Radio transmitters generally employ high level plifier stage, the positive voltage peaks will inor plate modulation for impressing the audio frecrease the plate voltage supplied to the amplifier 5 quency or modulation voltage upon the carrier tube by an amount equal to the magnitude of wave of the transmitter. In this type of modulathe modulation frequency voltage and conversely, tion, the plate circuit of the final stage of the the negative peaks will decrease the platevoltage. radio frequency power amplifier is subjected to At 100% modulation the plate voltage will rise to 10 the modulation voltage which is usually derived twice its normal value supplied by the plate 10 from the output of an amplifier. The latter amvoltage source at positive half, cycles of the moduplifies only the audio frequency currents for the lation frequency and to zero value at the negapurpose of modulation of the carrier and is gentive half cycle thereof. erally referred to as the modulation amplifier. A further object of this invention is to prevent In the system to which this invention is par-- the additive or the subtractive transfer of modu- 5 ticularly directed, a separate supply of operating lation voltage in excess to the magnitude of plate potentials is provided for the radio frequency am potential supplied irrespective of changes in the plifier to be modulated and for the modulation value of this potential. frequency amplifier; the coupling between the Prior arrangements have attempted to prevent two plate circuits being obtained by a coupling over modulation by a system of balance reached 20 circuit in which a suitable condenser isolates between the modulation voltage and the plate the direct current potentials of the two power voltage by means of auxiliary control of bias on supplies while providing a path for the modulacertain portions of the system. The resultant tion frequency currents. limiting of modulation peaks was effective only It is an established fact that by modulation of for one pre-set condition, and unbalance occurred 25 a carrier the amplitude thereof will vary in acas soon as the plate voltage changed in value. In cordance with the amplitude of the modulation other words, over-modulation was suppressed wave and at a. rate determined by the frequency only at a predetermined value of plate potential, of the latter. For full modulation when the posiand the system required rebalancing every time tive peak of the audio wave reaches its maximum, the plate supply voltage changed. 30 the peaks of the carrier wave become double the The particular feature of this invention is that Value reac e when 110 modulation s 0 0111'- .over-modulation is automatically controlled irring; and when the audio Wave reaches a a respective of variations in the plate voltage supmum in the negative direction, the carrier wave plied to the carrier amplifier to be modulated peaks are reduced to zero. T e P oportion bewithout requiring any balancing or setting of 35 tween the magnitudes of the unmodulated carcontrols. riel Wave and 'e h modulation frequency Another advantage resulting from the use of voltage determines the percentage of modulation. th means h r in proposed i that 100% modula- Thus t Percentage of l ion may be any tion may be maintained automatically at all times 40 value betwee Zero to 100 depending P the as long as the modulation voltage is equal or 40" amount of audio frequency applied to the modureater in value than the plate voltage supplied.

lated P' If the audio frequeny appned Additional objects and advantages will be apthereto 15 greater than necessary to obtal-n 100% parent from the following description of the inmodulation an over agi gi g g earner vention, defined particularly by the appended gz a g asgfigfiiggfi 3 3 22;; c Increases claims and taken in connection with the accom- The primary object of this invention is to pre- Paeymg w m ti 11 th r vent over modulation of the carrier, and to this Flgur? 1 Plllstmes Sc ema ca end means are provided for automatically limitmodulatlon ter operating on the posi we a ing the magnitude of modulation voltage transcycles of the modulatlon vqltage m 9 50 fer to the carrier frequency amplifying stage of P a Plate modulatedfiarner frequency the transmitter; I I fymg stag As stated before, in amplitude modulation sys- 2 s ow a mod ficatlon of t e l tm portems the positive peaks of the modulation fretion of Fig. 1 utilizing a double diode tube, the

6 5 q y W v e add d o e carrier wave amoperation of which includes both the positive and the negative half cycles of the modulation voltage; and

Fig. 3 shows an arrangement similar to Fig. 2 in which two individual diodes perform the same function.

Referring to Fig. 1, the final amplifier stage a. square, inasmuch as the components thereof have no direct bearing on this inventionand may be of the conventional type, comprising a master oscillator, a buffer and several other stages of amplification, depending upon the type of service for which the transmitter is designed. Only the final output stages which are to be modulated need be considered, of which the last stage is schematically shown. Continuing the description thereof, the input circuit is completed by the return of the secondary winding to the cathode 3, which is grounded, and in series therewith is a source of grid bias potential shown here for the purpose of illustration by the battery I. The secondary winding is tuned by the condenser 8 to the carrier frequency to be transmitted.

The filament of any of the tubes in the various figures of the drawing "is not completed to its source of heating current in order to simplify the illustration, for it is well known in the art that a, suitable source of heating current must be provided in order to operate tubes of the indirectly heated cathode type.

The output circuit ofthe tube I includes the plate or anode 9, the radio frequency load impedance Ill, which may be the primary winding of an output transformer II; the secondary I2 of which supplies the radiator or antenna l3.

The return terminal of the winding I is connected to the positive terminal of the plate potential source shown here for the purpose of illustration as the battery HI, the negative terminal of which is connected to ground. In series with the plate return lead is a modulation frequency reactance in the form of ,a choke coll I5. The condenser I6 by-passes the audio freqency energy to ground. v

The modulation frequency source is indicated by the output stage of an audio frequency amplifier which is shown as being fed from a microphone I! coupled to the primary winding I8 of the input transformer L9. The secondary winding 29 thereof excites the grids 2| and 2| .of the tubes 22 and 22', respectively. The output circuit of the above tubes is completed to the primary winding 23 of the push-pull output transformer 24 in the conventional manner, the plate potential source indicated by the battery 25 being connected between the junction of the cathodes 26 and 26' and the midpoint of the winding 23, the terminals of which are connected to the plates 27 and 21', respectively. The coupling circuit for the transfer of the modulation frequency voltage comprises the secondary winding 28 of the output transformer 24, one terminal of which is grounded and the other connected to theplate return side of the choke coil I through a coupling condenser 29.

The system so far described conforms in all its details with the commonly employed 'form of high level or plate circuit modulation. The novel feature of the invention resides in the particular connection and utilization of a unilateral current conductive device in the form of a diode tube 30, the anode 3| of which is connected to the high potential terminal of the winding 28 and the cathode 32 thereof to the positive terminal of the plate potential source It. In this manner, the space current path of the diode 30 is effectively connected in parallel with the reactance IS in so far as modulation frequency currents are concerned, the condenser 29 being of relatively low impedance for such currents.

Referring to the operation of the system, it will be seen that the modulation voltage to be transferred across the reactance l5 appears between the terminals of the secondary winding 28; that is, between ground and the high potential side thereof. This voltage is impressed across the reactance I5 through the condenser 29, and consequently when in the cyclic variation the plate terminal of the reactance I5 becomes positive, the voltage developed thereacross is in additive relation to'the voltage supplied by the battery I4, and when reversal of cycle occurs, the voltage is subtractive therefrom. Evidently,

when the two voltages are equal in magnitude,

the effective plate voltage for the tube I is doubled at one half cycle of the modulation voltage and reduced to zero at the nexthalf cycle thereof. Under this condition, 100% modulation is reached. It is to be noted that until such condition is obtained and at such condition of operation, the diode 30 remains non-conductive and has an infinitely high impedance. The reason for this is that its cathode 32 is always biased positive with respect to its anode 3| by the plate potential source I4. Hence as long as the modulation voltage is not greater than the voltage of the source I4 to counteract this bias, the diode forms no conductive path. However, should the modulation voltage exceed the value of the voltage supplied by the battery II, which may be the case by loud talking for instance, it the microphone II, the bias on the cathode 32 will not counterbalance the voltage on the anode 3|, and the tube 39 will draw current forming a conductive path effectively in parallel with the reactance I5. In this manner, the excess voltage results in a current by-passing the reactance and the energy is dissipated within the tube 30, whereas the modulation voltage remains of the value required for not more than 100% modulation.

to voltages not surpassing in magnitude the plate potential source irrespective of any changes in the latter. By this is meant that if the plate voltage source I4 decreases in value to say half its normal voltage, the bias on the cathode 32 of the diode 30 decreases also in proportion, and its conductivity is automatically adjusted to limit modulation frequency peaks to the-existing value of plate'potential.

Fig. 2 illstrates the modification of the coupling circuit wherein the limiting diode tube 30 is of the type having two plate anodes 3| and 3| connected in such manner that duo-lateral conductivity is obtained provided that the modulation voltage peaks exceed the value of the plate potential source M. In this case, both the positive and the negative peaks are resolved into currents finding a conductive path, and the subtractive transfer tending to lower the plate potential in a negative sense below the zero value is also limited and cannot be less than zero. In order to obtain operation of the tube in the manner stated, the output transformer 24 is provided with a mid-tapped secondary winding 28, each half of which is so proportioned in number of turns that the voltage across each secondary section is equal in magnitude to the voltage output of the single secondary winding of the transformer 24 shown in Fig. 1. 4

In order to obtain dual operation limiting voltage transfer in the coupling circuit with the arrangement shown in Fig. 1, twodiodes 30 and 30 may be connected in the manner shown in Fig. 3 in opposing relation of conductivity. As will be seen, the operation is identical with the one a coupling circuit therefor impressing said modulation frequency upon said stages and means for preventing overmodulation in said stages comprising a thermionic device, included in said coupling circuit and polarized .by said source of potential for limiting the magnitude of modulation voltagetransfer in said circuit.

2. In a modulation system of the high level type, a carrier frequency amplifier including a vacuum tube having an output circuit, a source of operating potentials supplying current thereto, a modulation frequency source and. a circuit interconnecting said output circuit therewith in-'- cluding said source of operating potentials, a thermionic device of unilateral conductivity ef fectively in parallel with said output circuit and polarized by said source of potentials whereby said device is rendered operative only upon potentials exceeding that of said last mentioned source.

3. In a modulation system of thehigh level type, a carrier frequency amplifier including a vacuum tuba-an output circuit between anode and cathode thereof includingan impedance and a source of anode potential, a modulation fre quency. amplifier, circuit means interconnecting said modulation frequency amplifier and said impedance whereby a modulation frequency voltage is transferred impressively across said impedance additively and subtractively with respect to said anode potential in cyclic variation with frequency and means for limiting the magnitude of said additive transfer automatically upon exceeding the value of said anode potential, comprising a shunt circuit effectively in parallel with said impedance, said last mentioned circuit including an element of variable current conductively operarier frequency vacuum tube amplifier having atleast a plate, cathode and control electrodes, an

output circuit between said plate electrode andsaid cathode including a carrier frequency impedance, a modulation frequency reactance and a source of plate potential in series; a modulation frequency amplifier having an output circuit including an impedance element, a coupling circuit interconnecting said impedance element and said reactance, said circuit including said plate potential source and a capacity in series, and a diode rectifier conductively connected between said impedance element and the plate terminal of said reactance.

5. In a plate circuit modulation system, a carrier frequency vacuum tube amplifier having at least a plate, cathode and control electrodes, an output circuit between said plate electrode and said cathode including a carrier frequency impedance, a modulation frequency reactance and a source of plate potential in series; a modulation frequency amplifier having an output circuit including an impedance element, a coupling circuit interconnecting said impedance element and said reactance, said circuit including said plate potential source and a capacity in series, and a diode rectifier having its anode conductively connected to said impedance element and its cathode to the positive plate terminal of said source.

6. In a plate circuit modulation system, a carrier frequency vacuum tube amplifier, a source of plate voltage therefor, a modulation frequencyamplifier, a coupling circuit for the transfer of modulation voltage from one to the other, and means in said circuit for limiting the magnitude of voltage transfer comprising a device of unilateral conductivity, said device being connected in said circuit and polarized by said source to form a current conductive path across said circuit upon application of voltage thereto exceeding a magnitude automatically predetermined by said source.

7. In a modulation system, a carrier frequency vacuum tube amplifier, a source of plate voltage therefor, a modulation frequency amplifier, a coupling circuit for the transfer of modulation voltage from one of said amplifiers to the other,

and means in said circuit for limiting the magnitude of voltage transfer comprising a device of duo-lateral conductivity, said device being connected in said circuit and polarized bysaid source to form a current conductive path across said circuit upon application of voltage thereto exceeding a magnitude automatically predetermined by said source.

8. In a plate circuit modulation system, a carrier frequency vacuum tube amplifier having at least a plate, cathode and control electrodes, an output circuit between said plate electrode and said cathode including. a carrier frequency impedance, a modulation frequency reactance and a source of plate potential in series; a modulation frequency amplifier having an output circuit including a, transformer with a divided output winding, a coupling circuit interconnecting said output circuit and said reactance, said coupling circuit including said plate potential source and a capacity in series, and a full-Wave rectifier having its anode connected to terminals of said divided output winding and its cathode to the positive terminal of said source of potential,

the midpoint of said winding being, connected to the negative terminal of said source.

9. In a plate circuit modulation system, a carrier frequency vacuum tube amplifier having at least a plate, cathode and control electrodes, an

' output circuit between said plate electrode and said cathode including a caiiier frequency im- 75 pedance, a modulation frequency reactance and a source of plate potential in series; a modulation frequency amplifier having an output circuit including an impedance element, a coupling circuit interconnecting s'aid impedance element and said reactance, said circuit including said plate potential source and a capacity in series, a diode rectifier having its anode conductively connected to said impedance element and its cathode to the positive plate terminal of said source, and a. second diode rectifier having its anode connected to the negative terminal of said source and its cathode to the plate terminal of said 5 

